The present invention relates to a brushless motor, and more particularly to a brushless motor composed of rotatable field poles, a plurality of armature windings disposed in confronting relation to the field poles and having winding frames defining spaces therein, and flux detector means such as Hall-effect devices positioned in the spaces, respectively, for repeatedly applying electric signals indicative of magnetic fluxes as detected by the Hall-effect devices to the other armature windings to enable the brushless motor to generate a torque in proportion to the magnetic fluxes.
There are widely used transistorized brushless DC motors in which the brush and commutator in ordinary DC motors are replaced with detectors for detecting rotor pole positions and transistor inverters. Dependent on the position of the rotor, the transistor inverters are energized or de-energized to pass currents through stator windings for generating torques between the stator windings and the rotor poles to rotate the rotor. The rotor pole position detectors may comprise proximity switches, photoelectric transducers, rotary transformers, or magnetism-sensitive devices such as Hall-effect devices. Where Hall-effect devices are utilized for detecting the rotor pole positions, the brushless motor is relatively small in size and hence used in a variety of arrangements which are required to be compact in overall size.
One known DC motor employing Hall-effect devices is illustrated in FIG. 1 of the accompanying drawings. The DC motor, generally designated by the reference numeral 8, of a two-phase construction has a rotor 2 composed of permanent magnets, two pairs of stator (armature) windings 4a, 4b and 4c, 4d disposed around the rotor 2, and a pair of Hall-effect devices 6a, 6c positioned adjacent to the stator windings 4a, 4c, respectively, and having four terminals. When the output currents from transistors energized by a transistor chopper are passed through the stator windings 4a, 4b and 4c 4d, the rotor 2 is rotated, and the Hall-effect devices 6a, 6c generate voltages based on the output currents from the transistors and the direction of the magnetic field produced by the rotor 2. The voltages generated by the Hall-effect devices 6a, 6c are useful for detecting the direction in which the rotor 2 rotates.
When the two-phase DC motor with the Hall-effect devices rotates at high speeds, the rotor is subject to a substantial inertial force and the output motor torque is rendered smooth. However, when such a DC motor with a relatively small number of phases is rotated at low speeds, no sufficient inertial force is produced and hence the output motor torque becomes irregular. This has led to a drawback that the DC motor requires complex controls.
In view of the foregoing deficiency, the applicant developed a brushless motor which is suitable for low-speed rotation, can produce an increased starting torque, and lends itself to accurate control for rotation. The applicant filed Japanese Patent Application No. 56-110170 entitled "Brushless motor having seven or more armature windings" based on the invention directed to the developed brushless motor. A U.S. Patent Application filed claiming Convention Priority based on the above Japanese Patent Application has already issued as U.S. Pat. No. 4,484,115. The applicant also filed Japanese Utility Model Application No. 57-186220 on a disk-type brushless motor. This disk-type brushless motor can be more compact and hold a current loss to a minimum for an improved motor efficiency. Since the brushless motor of this type is particularly suitable for low-speed operation and can easily be controlled, it would preferably be employed in an irradiation image recording and read-out apparatus utilizing a stimulable phosphor sheet, which the applicant has developed. More specifically, as disclosed in Japanese Laid-Open Patent Publications Nos. 55-12429 and 56-11395, for example, the stimulable phosphor, when exposed to an irradiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, stores a part of the energy of the irradiation. When the stimulable phosphor exposed to the irradiation is exposed to stimulating rays such as visible light, the stimulable phosphor emits light in proportion to the stored energy of the irradiation. For reading out an irradiation image stored in the stimulable phosphor sheet by exposing it to a laser beam, for example, the stimulable phosphor sheet is required to be fed along at a considerably low speed. The disk-type brushless motor is suitable for feeding the stimulable phosphor sheet at low speeds.
The polyphase brushless motor has an increased number of stator armatures with respect to field poles and Hall-effect devices disposed near the armatures. Although the rotor rotates smoothly at a low speed, it is not controlled highly efficiently in relation to the generated torque, i.e., the rotor is not rotated under the torque proportional to the magnetic flux produced.